1. Technical Field
An aspect of the present invention relates to a communication system having a base unit and at least one extension unit.
2. Related Art
Conventionally, a communication system having a base unit and extension units (for example, cordless telephones) has been widely used. The communication system may be configured such that information regarding the base unit and/or one of the extension units in use is indicated in the other extension units which are not in use. An example of such a communication system is disclosed in Japanese Patent Provisional Publication No. H5-336214. In the communication system, when the base unit is in use, for example, a button having a number “0” representing the base unit is illuminated in the extension units. When an internal communication is executed between two extension units (extension-to-extension call), a button having “1” and a button having “3” which respectively represent the two extension units are illuminated.
Thus, a user can recognize communication status (availability) of the base unit and the extension units which are in use based on the indication. When the communication is terminated, the illumination is ceased, and the user is notified of the termination of the communication.
In the communication system of the above configuration, however, information regarding the extension units which are not in use is not indicated, and the user cannot know the communication status of the other extension units.
Therefore, the user is only allowed to know the presence of the base unit and the extension unit currently in use as well as the presence of the extension unit in hand and kept away from seizing presence, communicability, and a number of additional extension units which are not in use.